Customisation has other major costs.
1: It locks you into a vendor. The vendor strongly encourages customisation for this reason (and for other reasons that are mostly against a business' interests)
2: onboarding staff is expensive because they need to learn your unique systems. If you're using a package/service off the shelf you can often advertise for people with knowledge of the package/service e.g. AWS versus inhouse e.g. known payroll system versus custom.
3: internal costs to upgrade. With stock standard systems your upgrades are cheap because the costs are amortised across many companies e.g. mobile app development in 2010's. With custom systems you pay for everything. The costs are often invisible e.g. horrific Mobile support (watching friends fight desktop websites on mobile is painful). Another cost is that internal development teams are often shit and certainly can never compete with Darwinian best-available-on-market software.
4: shitty custom internal systems. Sometimes the customisation provides business benefits but we have all worked with crappy internal systems where the benefits versus costs were slim.
Two examples of bad customisation requests:
A) HR systems in small companies. For some reason HR managers demand fractally complex systems and they all want different things so developing common features is really hard.
B) larger customers with demanding internal rules/regulations e.g. a client asking us to conform our UI to their UI standard. No way it was worth it for us or for them. Plus dealing with their UI despots would have been hell. I think their internal software teams decided to build inhouse instead (I bet the outcome was shit).